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Climate reconstruction using 44/40Ca isotopes in annually banded reef corals.

Author Chloé Pretet
Director of thesis SAMANKASSOU ELIAS
Co-director of thesis
Summary of thesis This report discusses the influence of temperature and salinity on calcium isotope fractionation in coral skeleton. Currently, the fractionation and calcium pathway in corals are not fully constrained. Biological (vital effects) or environmental parameters have influenced the isotopic composition. To better constrain the processes, the dataset is based on an accurate and high-resolution sampling coupled with a multiproxy approach. Samples were micro-drilled from fossil and modern tropical corals. The last deglacial fossil coral samples were cored during the IODP expedition 310 in Tahiti (Camoin et al., 2007). Ongoing analyses include modern corals cultured in monitored environmental conditions at different salinities (36, 38 and 40 ‰, Centre Scientifique de Monaco), following the method described in Reynaud-Vaganay et al. (1999). Ca and O isotopic composition as well as the Sr/Ca ratio were measured on the same sample. Ca isotope measurements are based on the method reported in Heuser et al. (2002) using a TIMS Finnigan Triton T1. Our preliminary results on fossil corals (Porites sp.) show that the fractionation is almost constant within the error bars. No relationship between precipitation rate and fossil coral δ44Ca is recognized. The weak temperature dependence reported in Böhm et al. (2006) is possibly not the only parameter that is responsible for the fractionation. Indeed, subseasonal temperature variations reconstructed by δ18O and Sr/Ca ratio are not mirrored in coral δ44Ca. Removing sea-surface temperature from the δ18O signal (δ18Osw) results in a coral δ44Ca curve that closely matches δ18Osw, pointing to salinity as an important factor that affects coral δ44Ca. However, one should note Cahyarini et al. (2008) were not able to reconstruct salinity variations based on δ18O and Sr/Ca in modern coral from Tahiti. To further constrain the influence of salinity on calcium isotope fractionation, current investigation focuses on cultured corals. Furthermore, we examine inter-colonies and inter-genus variability (Acropora sp., Porites sp., Montipora sp. and Stylophora sp.) to improve the understanding of calcium isotope behavior.
Administrative delay for the defence